This application is related to Japanese applications Nos. Hei 9(1997)-200371 and Hei 9(1997)-297440 filed on Jul. 25, 1997 and Oct. 29, 1997 respectively, whose priorities are claimed under 35 USC xc2xa7 119, the disclosures of which are incorporated by reference in their entirety.
1. Field of the Invention
The present invention relates to a bromine-impregnated activated carbon and a process for preparing the same. More particularly, the bromine-impregnated activated carbon is used for removing alkyl sulfides from gas containing the alkyl sulfides in small concentrations.
2. Description of Related Art
Sulfur compounds such as hydrogen sulfide, mercaptans and alkyl sulfides are contained in exhaust gas from sewage treatment plants, raw sewage treatment plants, waste disposal plants, and process gas and exhaust gas from chemical plants for petroleum refining, petroleum chemistry, paper pulp production and the like and from food processing plants.
Various methods for removing sulfur compounds contained in such gases are conventionally known, including an alkali absorbing method, a wet oxidation method, an ozone oxidation method, an activated carbon adsorption method and a combustion method, for example.
These conventional methods are useful for removing many kinds of sulfur compounds. However, alkyl sulfides such as dimethyl sulfide and dimethyl disulfide, which are specific smelly substances, cannot be removed to a satisfactory extent by the conventional methods.
Under these circumstances, Japanese Unexamined Patent Publications Nos. Sho 54(1979)-132470, Sho 50(1975)-130679 and Sho 55(1980)-51422, for example, propose that dimethyl sulfide and dimethyl disulfide can be removed by chemical adsorption by using a bromine-impregnated activated carbon instead of an usual activated carbon in the adsorption method. The chemical adsorption is a mechanism different from physical adsorption by the usual activated carbon.
Surely, in the case where the bromine-impregnated activated carbon is used, specific smelly substances such as dimethyl sulfide and dimethyl disulfide can be more effectively removed than the conventional activated carbon. However, the bromine-impregnated activated carbon differ in quality depending upon various parameters such as the amount of bromine carried by the activated carbon. Accordingly, in the present situation, dimethyl sulfide and dimethyl disulfide cannot always be removed to a satisfactory degree.
The present invention provides a bromine-impregnated activated carbon wherein the contents of any alkali metal, any alkali earth metal and iron are not higher than 0.3 wt % and the content of bromine is not lower than 3 wt %.
Further, the present invention provides a process for preparing the bromine-impregnated activated carbon comprising the steps of: reducing the contents of any alkali metal, any alkali earth metal and iron in a material activated carbon to 0.3 wt % or lower; and impregnating the activated carbon with bromine such that the content of bromine is not lower than 3 wt %.
In another aspect, the present invention provides a bromine-impregnated activated carbon, wherein the content of a surface oxide is not higher than 2.5 wt % in terms of oxygen and the activated carbon is impregnated with bromine such that the content of bromine is not lower than 3 wt %.
Further, the present invention provides a process for preparing the bromine-impregnated activated carbon of the above comprising the steps of: thermally treating a raw material activated carbon at 500xc2x0 C. to 1,100xc2x0 C. in the absence of oxygen to produce a carrier activated carbon; and impregnating the carrier activated carbon with bromine.
As a material activated carbon for producing the bromine-impregnated activated carbon of the present invention, usable is any activated carbon prepared from a raw material such as coal, coke, charcoal, coconut shell, resin, petroleum residuum or the like by a known process that has a specific surface area of about 100 m2/g to 2,000 m2/g. The material activated carbon may be in any form such as spherical, cylindrical, crushed, powdery, granular, fibriform or honeycombed form.
The alkali metals and alkali earth metals whose contents in the bromine-impregnated activated carbon of the present invention are controlled include Na, K, Mg, Ca, Zn and the like, for example. In whatever form these metals may be present, e.g., in a free form or in the form of a compound such as an oxide, a carbonate or a chloride, the contents thereof in terms of metals is preferably about 0.3 wt % or lower, more preferably about 0.1 wt % or lower, still more preferably about 0.05 wt % or lower, with respect to the weight of a material activated carbon. Especially, in the cases where the alkali metals and alkali earth metals are in a form which enables them to react with bromine, for example, in a free form, or in the cases where the alkali metals and alkali earth metals are ready to react with bromine, for example, in the form of an oxide (hydroxide) or in the form of an iodide, the contents of such metals are preferably about 0.2 wt % or lower in terms of the metals with respect to the weight of the material activated carbon.
The contents of the alkali metals and alkali earth metals are discussed here in wt % with respect to the material activated carbon. However, as described later, since the impregnation amount of bromine is usually within the range from about 3 wt % to about 50 wt % with respect to the material activated carbon, the contents of the alkali metals and alkali earth metals can be calculated, if necessary, in wt % to the bromine-impregnated activated carbon from the amount of bromine with which the bromine-impregnated activated carbon is impregnated. In this case, the contents of the alkali metals and the alkali earth metals are preferably about 0.3 wt % or lower, more preferably about 0.1 wt % or lower, still more preferably about 0.05 wt % or lower, for example, with respect to the weight of the bromine-impregnated activated carbon.
The content of iron in the material activated carbon is preferably about 0.3 wt % or lower, more preferably about 0.1 wt % or lower, still more preferably about 0.05 wt % or lower, and the content of iron in the bromine-impregnated activated carbon is preferably about 0.3 wt % or lower, more preferably about 0.1 wt % or lower, still more preferably about 0.05 wt % or lower. In the cases where the iron is in a form which enables them to react with bromine or in the cases where the iron is ready to react with bromine, the contents of iron is preferably about 0.2 wt % or lower in terms of the metals with respect to the weight of the material activated carbon.
Further, in the present invention, the bromine-impregnated activated carbon having the controlled contents of the alkali metals, alkali earth metals and iron as described above can exhibit more improved adsorption characteristics if the content of surface oxides is controlled as described below.
In the bromine-impregnated activated carbon of the present invention, the surface oxides whose content is controlled include various kinds of oxides such as carbon-oxygen complex (i.e., carbon monooxide, carbon dioxide), oxides of alkali metals and alkali earth metals. The content of such oxides is preferably about 2.5 wt % or lower, more preferably about 2 wt % or lower, still more preferably about 1.5 wt % or lower, in terms of oxygen with respect to the material activated carbon before the impregnation with bromine. In the bromine-impregnated activated carbon of the present invention, oxidative effect of bromine leads to oxidation of the activated carbon with time. As a result, the surface oxides tend to increase to some extent. Therefore, the content of the surface oxides in the bromine-impregnated activated carbon as finally obtained is controlled as discussed above in terms of weight with respect to the material activated carbon before the impregnation with bromine. In this case, the contents of the alkali metals and the like are preferably controlled, but the content of the surface oxides does not necessarily have relation with the contents of the alkali metals and the like.
The content of bromine in any bromine-impregnated activated carbon accordingly to the present invention is preferably about 3 wt % or higher, more preferably about 5 wt % or higher, still more preferably about 5 wt % or higher and about 50 wt % or lower, most preferably about 5 wt % or higher and about 20 wt % or lower, with respect to the material activated carbon before the impregnation with bromine.
According to the present invention, a method of reducing the contents of the alkali metals and alkali earth metals in the material activated carbon to about 0.3 wt % or lower is not particularly limited. Examples of such treatment methods include a method of dipping the material activated carbon in water or an acid solution, e.g., about 1 to about 10 wt % hydrochloric acid, phosphoric acid, nitric acid, sulfuric acid and organic acid (e.g., formic acid, acetic acid, oxalic acid and the like), and allowing to stand, stirring and/or boiling for a certain time period (e.g., for about 0.1 to about one hour); a method of feeding water or an acid solution into the material activated carbon for a certain time period (e.g., about one to about ten hours) with stirring the material activated carbon; and a method of passing water or an acid solution through a layer of the material activated carbon. These treatments may be carried out once or repeated twice or more. In the case where the material activated carbon is treated with the acid solution, the treatment is preferably followed by washing with water about one to about five times. In the case where the alkali metals and alkali earth metals can be removed (or deactivated) sufficiently only by washing with water, the contents of the alkali metals and alkali earth metals may be reduced to about 0.3 wt % or lower only by washing with water.
In the process for preparing the bromine-impregnated activated carbon of the present invention, in order to keep the content of the surface oxides in the bromine-impregnated activated carbon as finally obtained within the aforesaid range, the content of the surface oxides in the material activated carbon before the impregnation with bromine is preferably about 2.0 wt % or lower, more preferably about 1.5 wt % or lower, still preferably about 1.0 wt % or lower, in terms of oxygen.
In the process for preparing the bromine-impregnated activated carbon of the present invention, the content of the surface oxides in the carrier activated carbon before the impregnation with bromine may be reduced to about 2.0 wt % in terms of oxygen by thermally treating the material activated carbon at 500 to 1,100xc2x0 C. in the absence of oxygen, for example. This thermal treatment can be performed by various methods such as a high-temperature pyrolysis method and a high-temperature hydrogen reduction method. Alternatively, a liquid-phase reduction method may be utilized besides the thermal treatment in the absence of oxygen, for example. However, for controlling the content of the surface oxides in the material activated carbon efficiently, the high-temperature pyrolysis method and the high-temperature hydrogen reduction method are preferred.
The high-temperature pyrolysis method is a method of thermally treating the material activated carbon in an inert gas such as nitrogen gas, argon gas or the like which does not contain oxygen or in vacuum to thermally decompose oxides. As for temperature for the thermal treatment, the higher, the more efficient. However, since some activated carbons used as the material activated carbon have their pores closed at temperatures over about 1,200xc2x0 C., the thermal treatment temperature is suitably about 500xc2x0 C. to about 1,100xc2x0 C., preferably about 700xc2x0 C. to about 950xc2x0 C. A treatment time period can be decided as required according to the amount of the material activated carbon to be treated, and for example, the treatment time period is about 0.1 to about 10 hours.
The high-temperature hydrogen reduction method is a method of thermally treating the material activated carbon in an atmosphere of gas containing hydrogen to reduce oxides, for example. Temperature for this thermal treatment is suitably about 400xc2x0 C. to about 1,000xc2x0 C., preferably about 500xc2x0 C. to about 850xc2x0 C., more preferably around 700xc2x0 C. A treatment time period is suitably about 0.1 to about 10 hours.
The liquid-phase reduction method is a method of dipping or mixing/stirring the material activated carbon in a reducing solvent, for example, of hydrazine within the temperature range from room temperature to about 100xc2x0 C.
One of the above treatment methods may be performed once or repeated twice or more, or different ones of the above treatment methods may be combined in a multi-stage treatment.
Any bromine-impregnated activated carbon according to the present invention can be impregnated with bromine by a per-se known method. For example, the impregnation can be performed by a gas-phase impregnation method of contacting a carrier gas containing bromine gas with the activated carbon treated desirably, by a liquid-phase impregnation method of dipping the treated activated carbon in bromine water, and a spray impregnation method of spraying liquid bromine directly onto the treated activated carbon for impregnation.
Usually, in the gas-phase impregnation method, air, nitrogen gas, carbon dioxide, carbon monooxide or the like is used as the carrier gas, a contact temperature is set at about 150xc2x0 C. or lower, preferably about 80xc2x0 C. or lower, and bromine impregnation is continuously carried out using a fluidized bed, moving bed or spraying and mixing bed of the activated carbon, for example. In this method, after the activated carbon is impregnated with bromine by passing the bromine-containing gas, it is preferable to pass the carrier gas alone to remove remaining bromine gas.
In the liquid-phase impregnation method, the activated carbon is dipped in bromine water containing about 1% to about 5% bromine for about one to about ten hours. Or bromine water is passed through/contacted with the activated carbon using a fluid bed, moving bed, fixed bed or spray bed of the activated carbon to impregnate the activated carbon with bromine, followed by separation by filtration and drying. Temperature for this method is preferably about 80xc2x0 C. or lower, still more preferably about 50xc2x0 C. or lower.
In the method of spraying liquid bromine for impregnation, liquid bromine, bromine water or the like is sprayed onto the treated activated carbon with stirring the activated carbon, followed by drying, if necessary. Contact temperature for this method is preferably about 50xc2x0 C. or lower.
In order to adsorb/remove alkyl sulfides, especially dimethyl sulfide and dimethyl disulfide, from gas containing the alkyl sulfides with the bromine-impregnated activated carbon of the present invention, the gas to be treated may be contacted with the bromine-impregnated activated carbon by a known method such as a fixed bed, a moving bed, a fluid bed, a slurry-type or a batch-type contact stirring method, for example. A contact time period in this case can be varied as necessary according to the concentration of the alkyl sulfides, the particle size of the bromine-impregnated activated carbon used, the contact method and the like, but usually for about 0.1 seconds to about one minutes at about 50xc2x0 C. or lower.
Now described are preparation examples and application examples of the bromine-impregnated activated carbon of the present invention.
Activated carbon samples whose raw materials and properties are shown in Table 1 were dried in an air bath at 115xc2x0 C. for three hours by an electric drier, and then cooled to room temperature in a desiccator. The samples were collected 100 g each.
Subsequently, each of the activated carbon samples was put in a three-liter glass conical flask. A layer of the activated carbon was flattened, on which three grams of silica wool was put. Then, ten grams of bromine of guaranteed reagent grade was dropped onto the silica wool, and the flask was shook and stirred to impregnate the activate carbon with vaporized bromine gas. Thus, Samples A to C were prepared. The impregnation amount of bromine was calculated by an increase of the weight of the sample collected after all bromine got carried by the activated carbon and the gas within the flask turned colorless and transparent.
The bromine-impregnated activated carbons thus obtained were crushed and sieved with a 16 to 24 mesh and fed in glass columns of 15.6 mm inner diameter to form layers of 100 mm height (19.1 ml). Air having a relative humidity of 80% which contains 3 ppm dimethyl sulfide was passed at 40 cm/sec in the columns at 25xc2x0 C.
The concentration (C) of dimethyl sulfide in the air at an outlet of each of the activated carbon layers and the concentration (C0) of dimethyl sulfide in the air at an inlet to each of the activated carbon layers were measured by a gas chromatograph provided with an FPD detector. Time having passed until a breakthrough ratio C/C0 reached 0.05 was counted for each of the activated carbons (as a breakthrough time). The results are shown in Table 2.
For comparison, the breakthrough time concerning dimethyl sulfide was counted using the respective bromine-unimpregnated activated carbons (Axe2x80x2, Bxe2x80x2, Cxe2x80x2) in the same manner. The results are also shown in Table 2.
Ash of the respective activated carbons was mixed with lithium borate and melted, and then dissolved in dilute nitric acid. The contents of Na, K, Mg, Ca, Zn, Fe and Cu, which are likely to form bromides and usually are contained in activated carbons, were determined by Atomic Absorption Spectrometry. The results are shown in Table 3.